Resilient metallic sealing rings of C-shaped cross-section are widely known and used in numerous applications where relatively low leakage rates are allowable. To obtain a hermetic seal in vacuum and high pressure gas applications, however, it has heretofore been the common practice to reinforce the sealing force developed by the seal upon compression by the use of additional components, such as internal helically-wound toroidal "garter" springs (see U.S. Pat. No. 2,819,920) reverse facing concentric layers (see U.S. Pat. No. 4,218,067) or "resilience control members" (see U.S. Pat. No. 4,946,174).
FIGS. 2A and 2B depict a prior art C-shaped seals 1 and 2. Compression of the cross section of these seals induces meridional bending stresses reaching a maximum value on the axis of symmetry of the cross-section, indicated by the line labeled A. As compression increases from initial contact to a nominal 20% of the seal free or original height, where the maximum sealing contact force consistent with safe operation is obtained, the area of the cross-section in which the stress has surpassed the yield stress gradually extends until it approaches a fully-soaked condition over nearly the entire cross-sectional area at line A. Because only a small area of the cross-section, if any, at line A remains in an elastic state, the degree of springback obtained when the compression force is removed is reduced. A different shape of C-seal is shown in U.S. Pat. No. 3,879,043, which discloses a C-shaped seal having inwardly turned ends. However, the shape of these ends make them subject to crushing, also causing a reduction in springback.
One commonly used reinforced C-shaped seal is that which employs a helically-wound, toroidal "garter" spring nested inside the C-shaped cross-section of the sealing ring. An advantage of this arrangement is that it reinforces the sealing contact stress, thereby reducing leakage by increasing deformation of the seal material or coating at the sealing interfaces. A disadvantage is that in high pressure applications, the stiffness of the toroidal spring must be sufficient to overcome the stiffness of the C-shaped shell, which in turn must be thick enough to resist severe deformation and rupture by the pressure to be contained. The result is a seal with very little more springback than the plain C-shaped seal which it replaces and one which undergoes severe and debilitating stress relaxation--reducing both sealing force and springback--especially at elevated operating temperatures.
The importance of springback and the maintenance of sealing load are paramount in elevated temperature sealing of relatively flexible pressure containment structures such as rocket motors and jet engines. At operating pressures and temperatures, the joints sealed by resilient metallic seals usually experience a widening of the distance between their sealing faces, due to the effects of pressure forces and a reduction in the modulii of elasticity of their materials. Pressure-energization of segmental toroidal shell sealing elements provides partial recovery of their pre-compressed (pre-installed) dimensions, thereby tending to maintain the required sealing force. In many cases, however, this is insufficient to expand the seal cross-section beyond its natural springback recovery to ensure continued sealing as separation increases.